Discharge from grinding mills

ABSTRACT

A screening element ( 35 ) can be mounted at a discharge passage ( 28 ) of a grinding mill chamber ( 4 ). The discharge passage is positioned in use to receive ground particles moving thereinto in a discharge direction. The screening element comprises one or more openings ( 6 ) defined therein which are oriented such that ground particles pass therethrough in a direction that is oblique with respect to the discharge direction. The grinding mill chamber may form part of a centrifugal grinding mill.

TECHNICAL FIELD

This disclosure relates to the discharge of ground particles fromgrinding mills such as centrifugal grinding mills.

BACKGROUND TO THE DISCLOSURE

Centrifugal grinding mills are employed for the comminution of solidparticles, for example, mineral ores. Compared with tumbling mills(which are limited by gravitational acceleration), centrifugal grindingmills impart centrifugal acceleration to the mill contents as aconsequence of a gyrating motion, greatly enhancing the rate ofcomminution. The resultant higher velocity of mill contents can morereadily lead to blockages at discharge.

SUMMARY OF THE DISCLOSURE

In this disclosure there is firstly provided a screening element formounting at a discharge passage of a grinding mill chamber. Thedischarge passage is positioned in use to receive ground particlesmoving thereinto in a discharge direction. The screening elementcomprises one or more openings defined therein which are oriented suchthat ground particles pass therethrough in a direction that is obliquewith respect to the discharge direction.

The terms “oblique” and “obliquely” are to be construed herein asincluding the case where the ground particles can pass through the oneor more openings in a direction that is orthogonal with respect to thedischarge direction.

By orienting the one or more openings in a manner that results in anoblique direction of passage, it has been observed that the discharge ofground particles (ie. resulting from the milling of a feed to the mill)can be better facilitated and the blockage of openings in the dischargepassage, by eg. grinding media and/or oversize feed particles, can bemitigated, ameliorated or avoided. Hereafter, any grinding media or anyoversize feed particles present in the discharge passage willcollectively be referred to as “coarse particles”.

In one embodiment each of the one or more openings itself is orientedobliquely with respect to the discharge direction.

In another embodiment the screening element opening(s) are sized suchthat the coarse particles (including grinding media) are prevented fromdischarging from the passage whereas fine ground particles are permittedto discharge from the passage. The terminology “fine ground particles”refers to particles having a predetermined size to enable subsequent useand/or further classification. This predetermined size can be regulatedby controlling the size of screening element opening(s).

In one embodiment a plurality of openings are provided in the screeningelement.

In one embodiment the screening element comprises at least one annularplate mountable at the discharge passage, with a discharge passagebacking plate being mountable adjacent to but spaced from the annularplate that is most remote from the chamber. A screening element openingcan be defined between the backing plate and adjacent annular plateand/or between the chamber and annular plate adjacent thereto. In thisembodiment a plurality of annular plates can be mounted adjacent to butspaced from each other to define a plurality of openings therebetween.In addition, each of the backing and annular plates may be ofsubstantially lamina form.

In this embodiment, to define the screening element between the backingplate and the chamber, the one or more annular plates and the backingplate can be fastened together by one or more fixing elements. The oneor more fixing elements effectively clamp the assembly of one or moreannular plates between the backing plate and a wall of the grindingchamber, with the backing plate being positioned adjacent and clamped tothe annular plate most remote from the wall of the grinding chamber.

For example, each fixing element can comprise an elongate pin or boltextending through respective aligned holes in the one or more annularplates and the backing plate, with a proximal end (eg. head) of each pinor bolt being received at the backing plate, and a distal end of eachpin or bolt being mountingly received in an external wall of thegrinding mill chamber adjacent to the discharge passage. In this regard,the distal end of each pin or bolt can be externally threaded forengagement in a respective threaded recess in the chamber wall adjacentto discharge passage. The or each fixing element may alternativelycomprise a clamp or the like.

In this embodiment the one or more annular plates can be spaced fromeach other, and/or the backing plate and the annular plate adjacentthereto can be spaced from each other, by one or more respective spacerswhich define one or more screening element openings between adjacentannular plates and/or the backing plate and the annular plate adjacentthereto and/or between the chamber and annular plate adjacent thereto.

In this embodiment the spacers can be formed integrally with each plate.In another embodiment, the one or more spacers can be a plurality ofwashers discretely arranged in the space between and around theperiphery of the one or more annular plates. In this regard, to betterfacilitate spacer location, each spacer (eg. each washer) can be locatedon or at a respective elongate pin or bolt. Each spacer can then beclamped between adjacent plates.

The length of the screening element can thus be determined by the numberof annular plates and/or the size of spacers employed between thebacking plate and chamber wall. For example, spacers can be employedsuch that their thickness varies regularly or irregularly, to providefor control of and/or variation in the size of the or each screeningelement opening.

As a further variation, the plurality of annular plates can beconfigured such that their internal diameter can be varied regularly orirregularly. For example, moving away from the chamber, a progressivedecrease in the internal diameter of a plurality of annular plates canprovide a slope to an internal passage of the screening element, andthis slope can tend to cause coarse particles to be urged back into thegrinding chamber.

Thus, a variety of screening element internal passage shapes can beachieved, and these can be selected to maximise fine ground particlerelease and to cause coarse particle return to the grinding chamber.

In addition, the or each annular plate can be formed from a resilient orflexible material that can deform or flex under impact and thus causecoarse particles to be deflected back into the grinding chamber. Thedeformable or flexible annular plates also enable blockage clearance tobe effected without dismantling of the screening element.

Optionally or additionally, the spacers and/or fixing elements (eg.bolts or pins) can be formed from a resilient or flexible material thatcan be caused to deform or flex under impact on the or each annularplate and/or backing plate, again causing coarse particle return to thegrinding chamber and enabling blockage clearance to be effected withoutdismantling.

In addition, where the or each annular plate is formed from a resilientor flexible material, the plate(s) can deform or flex in accordance withdynamic forces imposed on these plates resulting from the in-use motionof the grinding chamber. This can induce a vibratory effect at the oreach annular plate which can assist with the transport of groundparticles through the screening element openings. In addition, thevibratory effect can inhibit or prevent entrapment of individual coarseparticles in the screening element openings, thus preventing blockage.

When a plurality of annular plates is employed the screening element canthus define a type of grate which has a series of openings, wherein eachopening can be oriented obliquely with respect to the dischargedirection.

Optionally, a grate can be defined in the backing plate, such that oneor more additional openings are provided. The additional openings may ormay not be oriented obliquely to the lateral direction. The additionalopenings can, for example, comprise a plurality of apertures of tubularform extending through the backing plate. In any case, in thisembodiment, the one or more additional openings may be sized such thatground particles can be released therethrough but coarse particlescannot.

Where additional openings are employed in the backing plate, they can beselectively configured to enable a predetermined discharge proportion offine ground particles moving through the discharge passage in adirection substantially parallel to the elongate axis of the dischargepassage. In this regard, the additional openings can be employed toallow for the discharge of some fine ground particles moving in thedischarge direction.

When, for example, the grinding chamber has a longitudinal axis, thedischarge direction can be represented as a vector that is inclined orgenerally orthogonal to the chamber axis in use. In yet anotherembodiment the grinding chamber has a longitudinal axis of symmetry, andthe discharge direction is generally inclined or orthogonal to this axisof symmetry in use.

In addition, the or each screening element opening may be oriented suchthat ground particles pass obliquely (eg. orthogonally) therethroughwith respect to the discharge direction vector. For example, where thedischarge passage defines an elongate axis therethrough, the dischargedirection vector typically aligns with this axis, and the groundparticles can pass through the or each screening element opening in adirection that is oblique (for example, normal) to the discharge passageelongate axis.

Further, where the discharge passage defines an elongate axis, the oreach annular plate and backing plate can be spaced apart along theelongate axis to define a series of annular screening element openingsof cylindrical form, symmetrical about the elongate axis.

Again, where the discharge passage defines an elongate axis, the radialthickness of the annular plates can reduce, moving outwardly from theelongate axis of the discharge passage. Accordingly, moving outwardlyfrom the elongate axis of the discharge passage, the transversedimension of the screening element openings can increase.

In an embodiment the elongate axis of the or each discharge passage maybe substantially normal to a wall of the grinding chamber.

In this disclosure there is secondly provided a grinding mill comprisingone or more screening elements as defined above. The grinding mill maybe a centrifugal grinding mill, for example, a centrifugal grinding millas defined hereafter.

In this disclosure there is thirdly provided a grinding mill comprising:

a grinding chamber;

a support for supporting the grinding chamber;

a feed passage in communication with the grinding chamber for directinginto the chamber a feed to be ground;

at least one discharge passage for receiving in a discharge directionground feed particles from the grinding chamber and for discharging theparticles therefrom;

a drive mechanism coupled to drive the grinding chamber in a manner thatcauses any grinding media and/or the feed in the chamber to grind thefeed and produce the ground feed particles; and

at least one corresponding screening element for mounting at thedischarge passage, the screening element comprising one or more openingsdefined therein which are oriented such that ground feed particles canpass therethrough in a direction that is oblique with respect to thedischarge direction.

The one or more screening elements may be configured as defined above.Furthermore, the one or more screening elements can be mountedexternally of the grinding chamber and disposed about a longitudinalaxis of the discharge passage. The one or more screening elements mayalso be configured to permit at least fifty percent of the fineparticles of ground product to discharge from the chamber in a directionsubstantially normal to the axis of the discharge passage.

In one embodiment the grinding mill is a centrifugal grinding mill andthe grinding chamber has a longitudinal axis of symmetry. Further, thegrinding chamber can be mounted to the support such that, when driven, anutating motion of the axis of symmetry about a relatively stationaryaxis of the grinding mill occurs, with the axes intersecting at a pointof nutation symmetry. In this embodiment the grinding chamber axis canbe inclined to and intersect with the axis of rotation of the chamber,to produce the nutating motion.

In this specification nutating motion of a machine element relative to afixed frame will be defined as the motion of an axis of the element suchthat it intersects with and traces out a conical surface about astationary axis of the fixed frame. In the general case, the nutatingelement has a net rotational motion about its axis, relative to thefixed frame. A special case of nutating motion is one in which thenutating element has not net rotational motion about its axis, relativeto the fixed frame. This special case can be achieved by employing atorque restraint mechanism, as described below.

In an embodiment the grinding chamber comprises a constraint mechanismhaving annular bearing surfaces which engage with complementary opposingbearing surfaces at the support. The opposing bearing surfaces may besymmetrical about the point of nutation symmetry, and can limit theamplitude of nutating motion.

In another embodiment the drive mechanism comprises a drive shaft havingan axis substantially co-linear with the stationary axis of nutatingmotion, with a proximal end of the drive shaft being driven by a powertransmission unit connected therewith. The drive shaft can be providedwith a cantilevered eccentric stub shaft mounted at its distal end andlocated adjacent to the grinding chamber. The stub shaft can have anaxis substantially co-linear with the axis of symmetry of the grindingchamber. The eccentric stub shaft can engage the grinding chamberthrough an intermediate bearing element adapted to permit relativerotational motion about the chamber axis of the stub shaft and grindingchamber.

In this disclosure this is fourthly provided a method of dischargingparticles from a grinding mill chamber, where the particles areinitially discharged from the chamber in a discharge direction, themethod comprising the step of altering particle direction once theparticles have discharged from the chamber to a direction that isoblique with respect to the discharge direction and then discharging theparticles.

In this method the particle direction can be altered by positioning ascreening element adjacent to where the particles are initiallydischarged from the chamber. The screening element can be adapted toreceive the discharged particles and may be an element as defined above.In this method the grinding mill chamber can form part of a grindingmill as defined above.

BRIEF DESCRIPTION OF THE DRAWINGS

Centrifugal grinding mills, including screening elements, will now bedescribed, by way of example only, with reference to the accompanyingdrawings in which:

FIGS. 1 and 2 are schematic sectional views of two known centrifugalgrinding mill arrangements;

FIG. 3 is a sectional view of a centrifugal grinding mill according to afirst embodiment;

FIG. 4 is an enlarged sectional view of constraint and restraintmechanisms of the grinding mill of FIG. 3;

FIG. 5 is an enlarged sectional view of the grinding chamber, showing ascreening module adapted to discharge fine product from the chamber;

FIG. 6 is an enlarged sectional view through the longitudinal axis of ascreening module;

FIG. 7 is a section through the screening module shown in FIG. 6, takenon a plane normal to the longitudinal axis, as indicated by Section X-X.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Referring firstly to FIGS. 1 and 2, cross sections through two knowncentrifugal grinding mills are shown. Screening element arrangements asdescribed below can be employed with either of the grinding millsdepicted in FIGS. 1 and 2, and may also be employed with other grindingmill arrangements.

Each of FIGS. 1 and 2 shows a grinding chamber 104, having an axis ofsymmetry 102, which rotates about a fixed axis 101, and intersectstherewith at a point of nutation symmetry 103. The chamber isconstrained to perform nutating motion by the engagement ofcomplementary annular bearing surface pairs 109 and 111, and 108 and110, which together form a bearing that is symmetrical about the pointof nutation symmetry 103. This bearing limits the amplitude of nutatingmotion.

Feed material, typically in the form of a dry coarse granular material131, or as a suspension of coarse particles in a fluid, or as acombination of both of these alternatives, is fed into the grindingchamber via the feed passage 105, and discharges from the oppositeextremity of the grinding chamber via openings 106, as a fine (orrefined) granular product. Screening element arrangements as describedbelow can be employed with any of the openings 106.

In the embodiment shown in FIG. 1, the grinding chamber 104 is drivenwith a nutating motion about stationary axis 101 by multiple pistons 159which are driven in phased sequence and bear against a flanged extensionof the grinding chamber located about the point of nutation symmetry103. In the embodiment shown in FIG. 2, the grinding chamber is drivenwith a nutating motion via a drive shaft 114, which is coupled toelectric motor 115 at one end, and engages with the end of chamber 104through an eccentric stub shaft 119 attached to the other end.

Rotation of the grinding chamber about its axis of symmetry may beprevented by a torque-restraining device connecting the grinding chamberto the support. For example, appropriate opposing gear plates can bearranged between the bearing surfaces 108,110 and 109,111 which meshtogether and thus prevent such rotation. Such an arrangement isdescribed below with reference to FIG. 4.

Typically, with the grinding mills depicted, the production of coarseand intermediate sized product material requires the use of grindingmedia 132. This media may have a typical size ranging from 5 to 20 mmspherical (or effective) diameter, which is typically larger than thesize of the discharge passages 106 in the wall of the chamber 104.Particles of grinding media, together with unground or partially groundfeed, are thus contained within the chamber, and only particles ofground feed material, or worn or small grinding media, having a sizesmaller than the openings 106, can discharge from the chamber.

Production of very fine product material, typically having 80 percentfiner than 40 microns, requires the use of correspondingly fine grindingmedia, typically 1 to 5 mm spherical (or effective) diameter, forminimum energy consumption. The use of correspondingly small dischargepassages 106 in the periphery of the grinding chamber 104 is notfeasible. It has been discovered that the known discharge passagesemployed with centrifugal grinding mills, for example those mills shownin FIGS. 1 and 2, are unsuited to production of very fine product. Forexample, as the size of discharge passage is reduced, the likelihood ofblockage by oversize particles increases inversely with the size ofdischarge passage. Furthermore, the very high surface pressures at theinternal surface of the grinding chamber are not compatible with thestructural or wear requirements of very fine discharge apertures.

Reference will now be made to FIGS. 3 to 7 in describing advantageousscreening element arrangements which can address, inter alia, issueswith blockage, overpressures, small grinding media etc, and provide forvery fine product material.

FIG. 3 shows a centrifugal grinding mill comprising a vertical axis ofrevolution 1 (ie. a stationary axis). A nutating axis 2 intersects axis1 at a point of nutation symmetry 3. A grinding chamber 4, symmetricalabout axis 2, is connected with a feed passage 5 at its upper end. Thefeed passage 5 intersects with an upper portion of the grinding chamberinner surface, and this intersection defines a plane which in turndefines an upper boundary to the grinding chamber, this boundary beinglocated below the point of nutation symmetry 3.

Grinding chamber discharge passages 28 extend through the peripheralwall of the grinding chamber, and each passage has a screening elementarrangement in the form of a screening module 35 mounted to the externalwall of the grinding chamber and adjacent to the passage. Each module 35has discharge openings 6 for discharging of fine product, as describedbelow.

A support for the grinding chamber comprises a frame member (or members)7 adapted to support the mill and transmit forces and moments generatedby the mill to suitable foundations. To determine the form of nutatingmotion of the grinding chamber constraints are provided in the form ofannular nutating bearing surfaces 8 and 9 rolling on corresponding fixedbearing surfaces 10 and 11, together with nutating and fixedpart-spherical surfaces 12 and 13 respectively having centres coincidentwith point of nutation symmetry 3.

A drive is located below the grinding chamber comprising a drive shaft14 adapted to be driven at its lower end by an electric motor 15 (orother power transmission device) through a flexible drive coupling 16.The drive shaft 14 is supported at its upper and lower ends by bearings17 and 18 respectively which are mounted in a support casing 22 in turnmounted to frame member 7. The drive shaft 14 is connected to aneccentric stub shaft 19, the stub shaft being mounted at its upper endto an underside of grinding chamber 4. In this regard, the stub shafthas an axis 20 that is held coincident with the nutating axis 2 of thegrinding chamber through the engagement of the stub shaft upper end withbearing 21, the bearing being mounted at the lower extremity of grindingchamber 4.

The grinding chamber 4 can be restrained from rotation about verticalaxis 1 by intermeshing bevel gears 29 and 30 disposed about the point ofnutation symmetry 3, as best shown in FIG. 4. Gear 30 fixed to grindingchamber 4, has nutating motion, and engages with stationary gear 29, totransfer (and thus restrain) torsional moment from the grinding chamberto the stationary frame 7.

Whilst a unitary chamber construction can be employed, in the embodimentshown in FIGS. 3, 5 and 6, the grinding chamber 4 is a multi-componentassembly, comprising upper casing members 23 and 24, and a lower chambermember 25, the members being secured together by fasteners (eg. bolts).Upper and lower casing members 23, 24 and 25 can be provided with innerreplaceable liners 26, 27 and 46, which are adapted to fit closelywithin the casing members, and protect them from damage by abrasive wearfrom grinding media, feed particles etc.

In addition, the lower chamber member 25 is itself a composite elementcomprising an outer metal structural casing 33 to which an elastomericmaterial is moulded and adhesively bonded thereto to form a thin innerlayer 34. Structural casing 33 is typically formed of a thin walledmetal, and is symmetrical about axis 2. The casing has a uniformcross-sectional shape transition from a substantially cylindrical format its upper edge, to a substantially circular planar form, normal toaxis 2, at its lower end. As a consequence of the thin wall of casing33, and its manufacturing method, the profile of this member issubjected to relatively high manufacturing tolerances. The mouldedelastomeric inner layer 34 is then intimately bonded to member 33 andprovides an accurate inner surface profile 47 for uniform engagementwith a replaceable chamber interior liner 46.

The members 23, 24 and 25 are structural elements, as they are requiredto absorb and transmit static and dynamic loads resulting from areaction to grinding chamber contents 32, drive loads, and inertialloading as a consequence of the nutating motion. On the other hand, theliners 26, 27 and 46 are non structural members, and are insteadselected to resist abrasive wear, and to protect structural casingmembers 23, 24 and 25 from the effects of wear, and to provide impactabsorption.

Referring now to FIGS. 6 and 7, cross sectional views through adischarge passage 28 and screening module 35′ are shown. The screeningmodule 35 is mounted to the external wall of grinding chamber 4, andengages within opening 40 in the chamber wall. Typically the module isfabricated such that its longitudinal axis is coincident with an axis ofdischarge passage 28. A screening module 35 may be mounted at eachdischarge passage 28 of the grinding chamber 4.

The screening module 35 comprises a body insert 36 of tubular form thatis shaped for snug positioning in opening 40. Body insert 36 has alength that enables it to project through the external wall of grindingchamber 4, to provide a lined discharge passage leading from linermember 46 and externally of the chamber.

Positioned in serial abutment to the insert 36 is a plurality of annularplates in the form of like annular disks 37. The annular disks aresandwiched (clamped) between insert 36 and a backing plate in the formof end plate 38, and are spaced from each other by a plurality ofspacers 41, to define respective discharge openings 6 therebetween. Themultiple discharge openings 6 thus have an annular-cylindrical form.

The maximum particle size of ground product discharged through openings6 is determined by the axial dimension of each discharge opening 6.Thus, variation in product particle size can be obtained by varying theaxial dimension of discharge openings 6. This can be achieved byreplacing the annular disks 37 with disks of appropriate geometry and/orby varying the dimensions of spacers 41. In addition, the total area ofdischarge openings 6 may be adjusted by varying the number of annulardisks 37 mounted in the screening module 35 and/or by varying thedimensions of spacers 41.

The spacers 41 may comprise projections of circular form defined on (eg.integral with) the surfaces of the annular disks 37. Alternatively, thespacers 41 may be provided as separate discrete elements located betweenadjacent annular disks 37, and may be of circular annular form (eg.washers). In either case, the spacers 41 provide for axial spacing ofthe annular disks 37 to define the discharge openings 6, and to enablethose openings to have closely controlled (or controllable) axialdimensions. In this regard, the spacer widths can be varied regularly orirregularly, to provide variation in the size of the discharge openings6 and thus axially variable particle (and particle size) discharge. Thisvariation may even impart a curved profile to the discharge passage.

In addition, the spacers 41 can be formed from a resilient or flexiblematerial that can deform or flex when particles impact on the annulardisks 37 or end plate 38. This deformation or flexing can allow forreflection of oversize particles back to the grinding chamber, and itcan allow for annular disk vibration and movement such that any blockagein the discharge openings 6 can be cleared (ie. without requiring moduledismantling).

To facilitate the sandwiching (clamping) of the annular disks 37, and tofasten the module together and to the chamber wall, fasteners in theform of four evenly spaced bolts 39 (or pins) are provided. The bolts 39engage and extend through respective aligned holes in the end plate 38,the annular disks 37 and the body insert 36, and into the external wallof grinding chamber 4. In this regard, a threaded end 39A of each boltis received in an internally threaded recess 42 of the chamber wall,whereas a bolt head 43 (or nut) urges against the end plate 38. Thebolts can be replaced with externally mounted clamps etc.

The bolts 39 and/or annular disks 37 can also be formed from resilientor flexible materials that can deform or flex when particles impact onthe annular disks 37 or end plate 38, and again this can result in diskvibration, causing reflection of oversize particles to the grindingchamber and the clearing of any blockages in the discharge openings 6.

Each of the annular disks 37 depicted in FIGS. 6 and 7 has a thicknessthat reduces radially outwardly. As shown in FIG. 6, this yields aclearance between adjacent annular disks that increases moving radiallyoutwards from the axis A. This increasing clearance assists with fineground particle release from the screening module, once the particlesinitially pass through openings 6 (ie. the openings flare outwardly,causing a pressure drop and hence partial vacuum that can draw particlesthrough opening 6). However, some or all of the annular disk surfacescan be planar and parallel to adjacent annular disk surfaces.

FIG. 6 also shows an end surface 50 of body insert 36 and an innersurface 52 of end plate 38 having a conical profile to mirror those ofthe adjacent annular disks. Again, these end and inner surfaces canalternatively be planar.

In an optional embodiment secondary discharge openings can be providedthrough end plate 38, to permit discharge of fine ground particles fromthe screening module in a direction substantially parallel to itslongitudinal axis. The secondary discharge openings can be of tubularform, and a variation in the product particle size discharged can beobtained by varying the aperture size of these secondary openings. As afurther alternative, end plate 38 can be perforated or grate-like. Thesecondary discharge openings allow for a proportion of the fineparticles travelling parallel to axis A to be discharged, but may thenhave the attendant problems of blockage.

Other embodiments may incorporate design variations to the embodimentshown in FIGS. 6 and 7 without affecting its principle of operation orperformance. For example, whilst FIG. 6 shows a constant aperture size(internal diameter) through adjacent annular disks, this aperture sizemay be varied regularly or irregularly. For example, moving away fromthe chamber, a progressive decrease in the aperture size of successiveannular disks can provide a slope to an internal passage of thescreening module, and this slope can tend to cause oversize particles tobe urged back into the grinding chamber. By having alternating largerand smaller aperture sizes differential disk vibration characteristicscan be induced, which may also assist with particle discharge.

Variations in screening module internal passage shape can thus beselected and optimised to maximise fine ground particle discharge andoversize particle return to the grinding chamber. Having stated this, byproviding a screening module internal passage shape that is cylindrical,the internal passage surface (ie. in which the discharge openings 6 arelocated) is thus substantially parallel to the discharge direction of ofthe grinding chamber contents (ie. feed material of varying degrees ofgrinding and any grinding media). Hence surface pressures (at theinternal passage surface) and associated abrasive wear effects may beminimised.

Because a relatively large number of discharge openings 6 can beprovided in the screening module 35, particularly favourable grinding tofine product sizes can be achieved and correspondingly narrow openings 6can be employed, without compromising product discharge. Known screeninggrates have incorporated a screen plate occupying an area nominallycorresponding with the diameter of a discharge passage (ie. with thegrate mounted transversely across the discharge passage). In the past,fine particle grinding has required very small openings in these screengrates, with the resultant chamber discharge not having high mechanicalstrength and being prone to blockage.

The screening module 35 allows for the available area of discharge to beincreased, without compromising mechanical strength. In addition, therelatively low surface pressures associated with the screening module 35results in reduced abrasive wear effects.

Further, the discharge of fine product material through openings 6 isassisted by the inertial effects of the nutating chamber motion, whichcan be operated at increased rates to dynamically expel the materialfrom the interior of screening module 35. This contributes to a highunit flowrate capacity of the screening module 35.

In operation of the centrifugal grinding mill shown in FIG. 3, driveshaft 14 is rotationally driven by motor 15, and this rotation isconverted to a nutating motion of the grinding chamber 4 by theeccentric stub shaft 19, the nutating motion being constrained byopposing bearing surfaces 8 and 10, and 9 and 11, and being disposedabout the point of nutation symmetry 3.

As a consequence of the nutating motion, inertial reactions from thenutating assembly are transmitted to the support frame 7, via stationarybearing surfaces 10 and 11. The drive assembly located below thegrinding chamber 4 is isolated from these inertial reactions by aresilient mounting through bearing 21.

Solid feed particles 31 are now fed into feed passage 5, where they fallunder gravity into grinding chamber 4. The feed particles interact(collide) with loose solid particles of grinding media 32, and withother coarse particles of feed material, with a gyrating and tumblingaction being imparted to all particles by the nutating motion of thegrinding chamber 4. This causes the feed particles to be broken (ground;comminuted) down to finer size fractions. In some grinding applications,separate particles of grinding media 32 are not used, and breakage ofparticles to finer sizes results from particle-to-particle, andparticle-to-wall interactions.

Fine size fractions of feed particles 31 are forced from grindingchamber 4 into discharge passage 28 in a discharge direction(approximately parallel to discharge axis A), and eventually the fineground particles discharge through discharge openings 6 in the screeningmodules 35.

Thus, ground material finer than the dimension of discharge openings 6passes therethrough, and material coarser than the dimension ofdischarge openings 6 is retained within (and typically returned to) thegrinding chamber 4 where it undergoes further size reduction.

The mill shown in FIG. 3 can be employed as a wet grinding mill in whichliquid, usually water, is also introduced into the grinding chamber 4,usually as a mixture with solid feed particles 31. Thus, fine productmaterial discharges from the screening modules 35 in the form of aslurry, and drains to a central sump 22 surrounding the drive shaft 14,from where it flows to a discharge pipe 42.

The centrifugal grinding mill can be adapted for use in dry grinding. Inthis case gas, usually air, is fed to the grinding chamber together withthe feed particles 31, and discharged product issues from openings 6 asa suspension or stream of fine particles in the gas.

The drive mechanism, bearings and other moving parts are sealedeffectively to prevent contamination from discharged product, whether inwet or dry form.

In the embodiment described, lubrication of bearings 17, 18, and 21 isprovided by lubricant which is continuously recirculated throughinterconnecting passages in the rotating elements, including shafts 14and 19. The recirculating lubricant provides cooling to remove anyexcessive heat generated in the bearings, and also removes contaminationresulting from bearing wear and entry of extraneous particles. Thelubricant discharging from the bearings may subsequently be filtered toremove contaminants, and may be cooled by heat exchange equipment asrequired, prior to recirculating it to the bearings.

It should be noted that the screening module can be employed withgrinding chambers in mills other than centrifugal grinding mills, andmay also be applied to a grinding chamber which is free to rotate aboutits axis (ie. axis 2). In an alternative application, the rotationalspeed of such a grinding chamber about its axis can be a smallproportion (eg. about two percent) of the nutating speed of acentrifugal grinding mill.

It should also be noted that other variations can be made to theembodiments described, as would be understood by a person skilled in theart.

1-39. (canceled)
 40. A screening element for mounting at a dischargepassage of a grinding mill chamber, the discharge passage beingpositioned in use to receive ground particles moving thereinto in adischarge direction, the screening element comprising one or moreopenings defined therein which are oriented such that ground particlespass therethrough in a direction that is oblique with respect to thedischarge direction.
 41. A screening element as claimed in claim 40wherein each of the one or more openings is itself oriented obliquelywith respect to the discharge direction.
 42. A screening element asclaimed in claim 40 wherein the screening element opening(s) are sizedsuch that coarse particles are prevented from discharging from thepassage whereas fine ground particles are permitted to discharge fromthe passage.
 43. A screening element as claimed in claim 40 wherein aplurality of openings are provided in the screening element.
 44. Ascreening element as claimed in claim 40 that comprises at least oneannular plate mountable at the discharge passage, with a dischargepassage backing plate being mountable adjacent to but spaced from theannular plate that is most remote from the chamber, and so as to definea screening element opening between the backing plate and adjacentannular plate and/or between the chamber and annular plate adjacentthereto.
 45. A screening element as claimed in claim 44 wherein aplurality of annular plates are mounted adjacent to but spaced from eachother to define a plurality of screening element openings therebetween.46. A screening element as claimed in claim 44 wherein each of thebacking and annular plates is of substantially lamina form.
 47. Ascreening element as claimed claim 44 that is defined between thebacking plate and the chamber, by fastening together the one or moreannular plates and the backing plate using one or more fixing elements,with the one or more fixing elements clamping the one or more annularplates between the backing plate and a wall of the grinding chamber. 48.A screening element as claimed in claim 47 wherein each fixing elementcomprises an elongate pin or bolt extending through respective alignedholes in the one or more annular plates and the backing plate, with aproximal end of each pin or bolt being received at the backing plate,and a distal end of each pin or bolt being mountingly received in thewall of the chamber adjacent to the discharge passage.
 49. A screeningelement as claimed in claim 48 wherein the distal end of each pin orbolt is externally threaded for engagement in a respective threadedrecess in the chamber wall adjacent to the discharge passage.
 50. Ascreening element as claimed in claim 44 wherein the one or more annularplates are spaced from each other, and/or wherein the backing plate andthe annular plate adjacent thereto are spaced from each other, by one ormore respective spacers which define one or more screening elementopenings between adjacent annular plates, and/or between the backingplate and the annular plate adjacent thereto, and/or between the chamberand annular plate adjacent thereto.
 51. A screening element as claimedin claim 50 wherein the one or more spacers are formed integrally witheach plate, or are a plurality of washers discretely arranged in thespace between and around the periphery of the one or more annularplates.
 52. A screening element as claimed in claim 51 wherein eachwasher is located on a respective elongate pin or bolt and is clampedbetween adjacent plates.
 53. A screening element as claimed in claim 50wherein the spacers are arranged such that their thickness variesregularly or irregularly, to provide for control of and/or variation inthe size of the or each screening element opening.
 54. A screeningelement as claimed in claim 44 wherein a plurality of annular plates isemployed and is configured such that their internal diameter is variedregularly or irregularly.
 55. A screening element as claimed in claim 54wherein, moving away from the chamber, a progressive decrease in theinternal diameter of the plurality of annular plates is employed toprovide a slope to an internal passage of the screening element.
 56. Ascreening element as claimed in claim 44 wherein the or each annularplate is formed from a resilient or flexible material that can deform orflex under particle impact.
 57. A screening element as claimed in claim44 wherein the or each annular plate can deform or flex in accordancewith dynamic forces imposed on the plates resulting from an in-usemotion of the grinding chamber, which induces a vibratory effect at theor each annular plate.
 58. A screening element as claimed claim 50wherein the spacers and/or fixing elements are formed from a resilientor flexible material that is caused to deform or flex under particleimpact.
 59. A screening element as claimed in claim 58 wherein the oreach screening element opening is oriented such that ground particlespass orthogonally therethrough with respect to the discharge directionvector.
 60. A screening element as claimed in claim 59 wherein, wherethe discharge passage defines an elongate axis therethrough, the groundparticles pass through the or each screening element opening in adirection substantially normal to the discharge passage elongate axis.61. A screening element as claimed in claim 60 wherein the or eachannular plate and backing plate is spaced apart along the dischargepassage elongate axis to define a series of annular screening elementopenings of cylindrical form that are symmetrical about the elongateaxis.
 62. A screening element as claimed in claim 61 wherein, movingoutwardly from the discharge passage elongate axis, the radial thicknessof the annular plates reduces, such that the transverse dimension of thescreening element openings increases.
 63. A screening element as claimedin claim 62 wherein the elongate axis of the or each discharge passageis substantially normal to a wall of the grinding chamber.
 64. Agrinding mill comprising one or more screening elements as defined inclaim
 40. 65. A grinding mill as claimed in claim 64 that is acentrifugal grinding mill.
 66. A grinding mill comprising a grindingchamber; a support for supporting the grinding chamber; a feed passagein communication with the grinding chamber for directing into thechamber a feed to be ground; at least one discharge passage forreceiving in a discharge direction ground feed particles from thegrinding chamber and for discharging the particles therefrom; a drivemechanism coupled to drive the grinding chamber in a manner that causesany grinding media and/or the feed in the chamber to grind the feed andproduce the ground feed particles; and at least one correspondingscreening element for mounting at the discharge passage, the screeningelement comprising one or more openings defined therein which areoriented such that ground feed particles can pass therethrough in adirection that is oblique with respect to the discharge direction.
 67. Agrinding mill as claimed in claim 66 wherein the at least one screeningelement is as defined in claim
 40. 68. A grinding mill as claimed inclaim 66 that is a centrifugal grinding mill, wherein the grindingchamber has a longitudinal axis of symmetry.
 69. A grinding mill asclaimed in claim 68 wherein the grinding chamber is mounted to thesupport such that, when driven, a nutating motion of the axis ofsymmetry about a relatively stationary axis of the grinding mill occurs,with the axes intersecting at a point of nutation symmetry.
 70. Agrinding mill as claimed in claim 69 wherein the grinding chamber axisis inclined to and intersects with the axis of rotation of the chamber,to produce the nutating motion.
 71. A grinding mill as claimed in claim69 wherein the grinding chamber comprises a constraint mechanism havingannular bearing surfaces which engage with complementary opposingbearing surfaces at the support, the opposing bearing surfaces beingsymmetrical about the point of nutation symmetry and being adapted tolimit the amplitude of nutating motion.
 72. A grinding mill as claimedin claim 69 wherein the drive mechanism comprises a drive shaft havingan axis substantially co-linear with the stationary axis of nutatingmotion, with a proximal end of the drive shaft being driven by a powertransmission unit connected therewith.
 73. A grinding mill as claimed inclaim 72 wherein the drive shaft is provided with a cantileveredeccentric stub shaft mounted at its distal end and located adjacent tothe grinding chamber, the stub shaft having an axis substantiallyco-linear with the axis of symmetry of the grinding chamber, with theeccentric stub shaft engaging the grinding chamber through anintermediate bearing element adapted to permit relative rotationalmotion about the chamber axis of the stub shaft and grinding chamber.74. A method of discharging particles from a grinding mill chamber,where the particles are initially discharged from the chamber in adischarge direction, the method comprising the step of altering particledirection once the particles have discharged from the chamber to adirection that is oblique with respect to the discharge direction andthen discharging the particles.
 75. A method as claimed in claim 74wherein the particle direction is altered by positioning a screeningelement adjacent to where the particles are initially discharged fromthe chamber, the screening element being adapted to receive thedischarged particles and being as claimed in claim
 40. 76. A method asclaimed in claim 75 wherein the grinding mill chamber forms part of agrinding mill as defined in claim 66.